In recent years, there is growing interest in a lean burn engine for improving the fuel consumption of automobiles, and various types of lean burn engines have been proposed, for example, as described in Japanese Patent Laid-open No. Hei 6-88562.
A lean burn engine is not necessarily operated in a lean burn state over the entire operating region but is operated at an air-fuel ratio switched between a lean ratio state and a stoichiometric ratio state in accordance with an operational condition.
The lean burn engine described in the above document, Japanese Patent Laid-open No. Hei 6-88562, is operated at an air-fuel ratio switched from a lean ratio state to a stoichiometric ratio state when an engine load is more than a load limit in the lean ratio state. The load limit in the lean ratio state is lower than that of the stoichiometric ratio state. The reason for this is as follows:
Since a necessary fuel amount is little changed when an engine load or an engine torque is not changed, a necessary intake air amount is increased linearly with an air-fuel ratio at the same engine torque. Accordingly, the lean ratio state requires an air mount larger than the stoichiometric ratio state does.
On the other hand, the limit of the maximum intake air amount upon full open of a throttle valve is determined on the basis of an engine speed because the engine structure is not changed, so that the operable load limit in the lean ratio state is made smaller than that in the stoichiometric ratio state. The engine is not operated in the lean ratio state when the engine load is more than the load limit in the lean ratio state. As a result, in the related art, when the engine load is more than the load limit in the lean ratio state, the engine is operated in the stoichiometric ratio state switched from the lean ratio state. Such a technique fails to sufficiently take into consideration the enlargement of the lean burn operating region, and to obtain the fuel consumption of an engine expected to be improved due to lean burn operation of the engine.
Namely, in the related art, since the lean burn operation is directly switched to the stoichiometric burn operation when an engine load is more than a load limit in the lean ratio state, the ratio of the stoichiometric burn operation to the lean burn operation becomes large in the case of an engine of an automobile, that is, in the case where a load constant state such as cruising operation and a high load state such as accelerating operation are frequently repeated. As a result, it fails to sufficiently improve the fuel consumption of an engine due to lean burn operation.